Ruthenium or Osmium based catalysts already play an important role in the homogeneous hydrogenation reactions of various substrates for many years as summarized in the Handbook of Homogeneous Hydrogenation, 2007, Volume 1, Pages 45-70 (Edited by De Vries, Johannes G.; Elsevier, Cornelis J).
In WO99/26949A1 and Organometallics, 1998, 17(16), 3460 ruthenium and osmium based complex catalysts have been disclosed which contain bidentate Schiff-Base ligands. It is reported that such complex catalysts are suited for olefin metathesis. It further on described that these ruthenium or osmium based Schiff Base complexes are highly stable to air, moisture and temperature and even exhibit catalytic activity in polar protic solvents. Said (O, N) bidentate Schiff base ligands were recognized as especially feasible for fine-tuning of ligand parameters since their steric and electronic environment can be tailored by the proper choice of the starting materials, i.e. the amine and salicyl aldehyde type. The ruthenium based complexes can be prepared by the treatment of RuCl2(═CHPh)(PCy3)2 (3), the so-called Grubbs (I) catalyst, with a variety of Schiff-base ligands as thallium salts as shown in the following Scheme in which the compound numbering given below the formulae is the same as used in Organometallics, 1998, 17(16), 3460.
wherein R1 and R2 have the following meanings in compounds 8a-h:8a R1=H, R2=2,6-i-PrC6H3 8b R1=4-NO2, R2=2,6-i-PrC6H3 8c R1=4-NO2, R2=2,6-Me-4-MeOC6H2 8d R1=4-NO2, R2=2,6-Me-4-BrOC6H2 8e R1=4-NO2, R2=2,6-Cl-4-CF3C6H2 8f R1=6-Me-4-NO2, R2=2,6-i-PrC6H3 8g R1=4-NO2, R2=2,6-i-Pr-4-NO2—C6H3 8h R1=4-NO2,

However, at metathesis temperatures around room temperature these ruthenium based Schiff Base complexes have a lower activity compared to Grubbs (I) catalyst which is RuCl2(═CHPh)(PCy3)2, because they are highly thermally stable under standard catalytic reaction conditions. They only show a higher activity in ring-closing metathesis (RCM) reactions when these are performed at elevated temperatures as e.g. 70° C.
Chem. Rev., 2010, 110(3), 1746-1787 encompasses a review by G. C. Vougioukalakis and R. Grubbs that Schiff base N,O-bidentate ligands can be also introduced into NHC ligand containing Ru-based complexes (with “NHC” standing for a N-heterocyclic carbene ligand) and that the resulting complexes can act as catalysts for atom transfer radical polymerization (ATRP), ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), olefin isomerisation, enol ester synthesis and Kharasch addition. The Ru-based NHC-Schiff base metathesis catalysts disclosed have for example the following structures wherein the numberings given below the formulae are the same as used in Chem. Rev., 2010, 110(3), 1746-1787.
wherein R1 and R2 have the following meanings in compounds 343 to 349:343 R1=CH3, R2=H344 R1=CH3, R2=NO2 345 R1=2,6-(CH3)2-4-BrC6H2, R2=H346 R1=2,6-(CH3)2-4-BrC6H2, R2=NO2 347 R1=2,6-i-Pr2-C6H3; R2=H348 R1=2,6-i-Pr2-C6H3; R2=NO2 349 R1=2,4,6-(CH3)3—C6H2, R2=H
The catalysts of the following formulae are commercially available from Umicore as catalysts “M41” and “M42” and are recommended for metathesis reactions:

In Angewandte Chemie International Edition 2003, 42(25), 2876 T. Opstal and F. Verpoort have disclosed that the two donor atoms of Schiff base ligands in ruthenium based complexes have opposing natures: the phenolate oxygen atom is a hard donor and known to stabilize the higher oxidation states of ruthenium, whereas the imine nitrogen atom is in comparison soft and a stabilizer of the lower oxidation states. It was recognized that combining an N-heterocyclic carbene ligand carrying mesitylene groups as substituents of the two N atoms in the ligand with a bidentate Schiff base ligand in the same compound should lead to superior performance, because    1) the strong electron-donating ability of the imidazol-2-ylidene-type ligand, i.e. the N-heterocyclic carbene ligand, can facilitate the decoordination of one side of the bidentate ligand plus it can stabilize the generated intermediate;    2) the bulky mesitylene groups protect the second carbene species in the complex and prevent bimolecular decomposition; and    3) with a phosphane-free catalyst, P—C decomposition reactions are avoided.
However, it is further reported that besides the increased stability, the association of both, the bidentate Schiff base ligands and NHC ligands into Ru complexes also causes rather low catalytic activity at lower temperature. To achieve an activation of this type of “latent” Ru—NHC-Schiff Base catalysts, specific protocols, e.g. heating or introduction of acidic co-catalysts (HCl, BF3, SiCl4, HSiCl3 etc.) are needed as e.g. reported in J. Polym. Sci. A Polym. Chem., 2010, 48(2), 302, as well as Dalton Trans, 2007, 44, 5201.
In Adv. Synth. Catal., 2007, 349(3), 395-404 it is disclosed that catalysts bearing both bidentate salicylaldimine ligands and NHC ligands represent robust olefin metathesis catalysts and test data has been provided for successful ring-closing metathesis of dienes and enynes in methanol and methanol/water mixtures under air. Examples of the investigated catalysts are shown in the following wherein the compound numberings given below the formulae are the same as used in Adv. Synth. Catal., 2007, 349(3), 395-404. The synthetic routes again employ the Schiff base ligands as highly toxic Tl(I) salts.

In Journal of Organic Chemistry 2007, 72, 3561 a Tl salt free and efficient synthesis of bidentate Schiff Base ligand containing ruthenium based complex catalysts for olefin metathesis is described. The new synthesis involves a two-step, quasi-one-pot approach and Ag(I) carbonate as base as shown in the following scheme wherein R1 and R2 may different meanings as disclosed in Journal of Organic Chemistry 2007, 72, 3561.

Although different Ruthenium based complex catalysts containing a NHC-ligand and a Schiff base type ligand have been reported in the prior art so far, they all have one common structural feature: besides the NHC ligands and Schiff base ligands, they all additionally contain a Ru-alkylidene structure [Ru═C—]. So far these Ru-based NHC-Schiff base catalysts are mainly disclosed as catalysts for atom transfer radical polymerization (ATRP), ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), olefin isomerisation, enol ester synthesis and Kharasch addition.
In JP 2002/030057A new ruthenium based complexes having a Schiff base ligand as shown below were synthesized and isolated.

These complexes have been reported as being capable of exhibiting a higher catalytic activity for partial hydrogenation of a cyclic polyene such as cyclododecatriene to cyclododecene than that of a conventionally known catalyst system without using an additive which is indispensable to conventional ruthenium complex catalyst systems. JP 2002/030057A does not contain any disclosure or teaching whether or not such catalysts are suited and active for the hydrogenation of other types of compounds or polymers with high conversion rates and if yes under which conditions and with which activity.
In Transition Metal Chemistry 2004, 29, 644-648, an efficient method for introducing a bidentate Schiff base ligand into Ru complexes is described which yields new hexa-coordinated Schiff base complexes of the type [RuCl(CO)(EPh3)(B)(L)] with E being P or As, B being PPh3 or AsPh3 or pyridin or piperidine and L being the anion of the Schiff bases derived from 2-hydroxy-1-naphthaldehyde and aniline, 4-chloroaniline or 2-methylaniline, respectively. This synthesis route operates in the absence of a metal catalyst mandatory pursuant to a number of other prior art references. The antibacterial activity of these complexes was investigated, but no catalytic properties were investigated or mentioned. The preparation process disclosed is shown in the following for E representing phosphorus, and B as well as L both being PPh3.

In Inorganica Chimica Acta 358, 2005, 3218-26, a complex RuHCl(CO)(PPh3)3 is converted with 2,1,3-benzothiadiazole (“BTD”) to prepare the complex RuHCl(CO)(PPh3)2(BTD). A Schiff base is then added to the complex RuHCl(CO)(PPh3)2(BTD) to remove the BTD ligand in order to introduce the Schiff-base ligand to the Ru/Os metal center as shown in the following scheme. When the complex RuHCl(CO)(PPh3)2(BTD) is reacted with the Schiff-base ligand, the obtained Ru Schiff-based complex is generated in microcrystalline form which can be easily (re)crystallized from the reaction system with very good purity.

As the hydrogenation of compounds is of high importance it was the object of the present invention to provide access to novel ruthenium or osmium based complexes which are excellent hydrogenation catalysts on the one hand and which show a high stability in air or water on the other hand. In particular such complexes should dispose of a high hydrogenation activity for large-scale industrial use at ambient reaction conditions. Preferably such novel complexes should exhibit a high hydrogenation activity even at low reaction temperatures. With regard to the hydrogenation of nitrile rubber the current industrial processes often use expensive Rh-based catalyst systems like Wilkinson's catalyst together with PPh3 as co-catalyst. After hydrogenation, extra time and costs must be spent to remove and recycle the expensive Rh-based catalyst. It would be desirable to find a catalyst disposing of such a high activity that no co-catalyst needs to be used anymore on the one hand and recovery and recycling of the catalyst would no longer be necessary on the other hand. This would reduce catalyst costs as well as process costs substantially. Compared to hydrogenated polymers as obtained nowadays with known catalysts it is important that the hydrogenated polymers to be obtained by using any new complex catalyst must not show any changes in polymer properties and the polymer's vulcanization behaviour as this has been unfortunately observed for other catalysts.
This object has now been solved by providing novel ruthenium or osmium based complex catalysts containing on the one hand a Schiff-base type ligand and on the other hand an N-heterocyclic carbene (“NHC”) ligand, but no Ru- or Os-alkylidene structure.